In some circumstances, yes, but in general, no: game theory doesn't say that.
If the coalition's large parties have alternative small parties to choose from, the smallest party has to compete for entrance with the other smallest parties.
This 'reverse auction' makes it possible that a small party would end up with net 0 power when the right to govern is taken into account (could be itself negative).
An agreement that excludes a new election from being called should be possible at this point (e.g. Conservative-Liberal Democrat coalition agreement).
The fact that we normally observe smallest members having outsized power might be explained by, e.g.:
1) Coalition in a two/three-party Westminster-style system tends to mean the third party is essential and doesn't have competition
2) Regression to the mean: we would observe this anyway if power in coalition was independent of party size.
If the coalition's large parties have alternative small parties to choose from, the smallest party has to compete for entrance with the other smallest parties.
This 'reverse auction' makes it possible that a small party would end up with net 0 power when the right to govern is taken into account (could be itself negative).
An agreement that excludes a new election from being called should be possible at this point (e.g. Conservative-Liberal Democrat coalition agreement).
The fact that we normally observe smallest members having outsized power might be explained by, e.g.: 1) Coalition in a two/three-party Westminster-style system tends to mean the third party is essential and doesn't have competition 2) Regression to the mean: we would observe this anyway if power in coalition was independent of party size.